At present, motor vehicle heat exchangers are leak-tested using various possible methods, which are selected as a function of the type of leak that is being looked for.
A first method consists in immersing the heat exchanger in a liquid such as water, in putting the air in its internal cavity under pressure, and then looking for the appearance of bubbles leaving the heat exchanger and passing through the liquid. The quality of the results achieved depends significantly on the attention of the operator. Parts that have been tested need subsequently to be dried, thereby lengthening the inspection cycle. The operations performed are not traceable.
In a second method, a determined excess pressure is established inside the internal cavity; the heat exchanger is closed and the rate at which its internal pressure varies over time is measured. If it has a leak, then the pressure drops, and by measuring the pressure drop it is possible to deduce the magnitude of the leak. That method is of limited sensitivity. A test time of about 30 seconds is required for testing a part whose inside volume is one liter and to obtain sensitivity of a few cubic centimeters per minute. The result also depends on variations in temperature and in the volume of the heat exchanger.
In a third method, a helium test is used. For this purpose, the heat exchanger for testing is placed in a leakproof enclosure connected to a helium leak detector, and the heat exchanger is connected to pressurization apparatus of the kind shown in FIG. 1. Air pressure of the order of 2.times.10.sup.3 hPa to 10.times.10.sup.3 hPa, depending on the type of heat exchanger, is then established inside the internal cavity of the heat exchanger, and major leaks are looked for by measuring variation in the internal pressure of the heat exchanger. If there is no major leak, the pressure inside the heat exchanger is returned to atmospheric pressure, and then the inside air is evacuated to a low pressure, e.g. about 10 hPa, after which helium is introduced into the inside cavity of the heat exchanger up to a pressure of about 10.sup.3 hPa. Major leaks are then looked for by using a helium leak detector to detect the temporary presence of helium around the motor vehicle heat exchanger in the leakproof test chamber. Detection must be performed for helium concentrations that are much greater than the natural concentration of helium in the air. Thereafter, helium pressure of about 2.times.10.sup.3 hPa to 10.times.10.sup.3 hPa is established inside the heat exchanger and small leaks are looked for by detecting the presence of helium, if any, around the motor vehicle heat exchanger in the test chamber, again at a level that is significantly greater than the natural concentration of helium in the air. Thereafter the pressure inside the internal cavity of the heat exchanger is reduced to below atmospheric pressure, the heat exchanger is disconnected, and it is withdrawn from the test chamber.
Amongst the above-mentioned methods, the helium test is the most reliable, the most accurate, and the most sensitive. Nevertheless, it suffers from various drawbacks. Firstly, it is necessary to supply the helium tracer gas, at concentrations generally of the order of 10% to 100%, and this gas is consumed. That considerably increases the cost of the test. Thereafter, after massive exposure to helium, it is necessary to leave the machine unoccupied for as long as it takes to enable it to be depolluted. Also, the test is not entirely reliable since it often leads to acceptable parts being wrongly rejected. What happens is that when a defective heat exchanger having leaks is tested, helium escapes into the test chamber. After the defective heat exchanger has been withdrawn, residual helium can remain in the test chamber, thereby disturbing subsequent leak measurements on another heat exchanger, and giving the impression that said other heat exchanger also leaks.